Saturable core apparatus



6, 1957 G. H. DEWITZ 2,802,186

S ATURABLE CORE APPARATUS Filed April 19, 1952 INVENTOR.

GZ'RHARD H. DEW/T2 2,8@Z,l86 Patented Aug. 6, 1957 SATURABLE CORE APPARATUS Gerhard H. Dewitz, Westport, Conn, assignor to C G. S. Laboratories, ind, Stamford, Conn., a corporation of Connecticut Application April 19, 1952, Serial No. 283,136

13 Claims. (Cl. 336-471) frequency is increased. With increasing frequency, the

hysteresis, eddy current, and dielectric losses within the ferromagnetic material increase sharply.

It is among the objects of the present invention to extend the high frequency range, increase the efficiency, improve the operating characteristics, reduce the core losses and increase the effective Q of high frequency saturable core magnetic apparatus.

In using saturable core ferromagnetic apparatus at high frequencies and particularly in extending the upper limit of the frequency range to higher frequencies, one of the important factors is the maintenance of relatively low core losses. By maintaining a relatively low core loss in the high frequency range, such apparatus can be used over a wider range of different frequencies with a good frequency response throughout the whole range, and it is more sensitive so that increased power gain is available, that is, greater changes in the inductance of the controlled winding and greater variations in the output signal are obtained with a given control signal.

The present invention will be described as embodied in an improved magnetic modulator having a pair of separate ring cores with a control winding and a controlled winding around both of them. The ring cores are formed of ferromagnetic ceramic material, as will be described later.

A layer of electrically conductive material in the form of r a conductive shield is placed between one or more of the windings and the core material and substantially increases the Q of the windings and extends the frequency range. In one embodiment, the conductive shield is formed by a layer of conductive paint sandwiched between layers of insulating film, in another embodiment the surfaces of the core material are painted with a layer of conductive paint.

These and other aspects, objects and advantages of the present invention will be in part pointed out and in part apparent from the following description considered in conjunction with the accompanying drawings, in which:

Figure 1 is a diagrammatic perspective view of a magnetic modulator, generally indicated at 2;

Figure 2 is a schematic circuit diagram of one method of connecting the magnetic modulator 2;

Figure 3 is a cross sectional view of one side of the modulator 2 taken along the line 33 in Figure 1;

Figures 4 and 5 are views similar to Figure 3 showing other shielding arrangements;

Figure 6 is a top view of a multiple ring core structure showing another shielding arrangement; and

Cir

an (2U Figure 7 is a sectional view taken along line 77 of Figure 6,,the windings being omitted to simplify this view.

The magnetic modulator 2 includes first and second ring cores 4 and 6, respectively, formed from a ferromagnetic ceramic or ferrite material, for example of the kind described by Snoek in U. S. Patents, 2,452,529; 2,452,530; and 2,452,531.

A signal winding, generally indicated at 8, consists of two series winding portions 10 and 12, respectively wound on the cores 4 and 6. The ends of this winding 8 are connected to terminals 14 and 15. The winding portions 10 and 12 are wound in opposite directions or connected so as to cancel out any flux linkage and to minimize in-- ductive coupling between the winding 8 and a control winding 16 which encompasses both cores 4 and 6.

The control winding 16 is connected to two terminals 17' and 18 by which a control signal is applied to this winding in order to vary the magnetic saturation of the cores 4 and 6, thus modulating the signals flowing through the winding portions 10 and 12.

One possible arrangement for connecting the magnetic modulator 2 is shown in Figure 2. The portions 10 and 12 of the signal winding 8 are connected in series and a high frequency signal to be modulated, for example such as a radio frequency signal from a signal generator 19, is fed through the winding 8 into a radio frequency load circuit, diagrammatically indicated at 20.

As pointed out above, in order ot minimize inductive coupling between the signal or controlled winding 8 and the control winding 16, the portions 10 and 12 of the signal winding 8 are connected so that the voltages which they each induce in the winding 16 are equal and opposite and so cancel. A control current is applied through the terminals 17 and 18, to the control winding 16. Any change in the magnitude of this control current changes the saturation of the cores 4 and 6 and hence changes the inductance of the winding 8. These variations in the inductance of the winding 8 in turn control the high frequency signal current flowing through the load 2%. Thus, the output through the load 20 is modulated in accordance with the fluctuations of the control voltage.

As shown in Figure 3, a layer 22 of electrically conductive material forming an electrostatic shield but not a shorted turn is placed between the winding portions 10 and 12 and the control winding 16. This shield may be formed of conductive foil or a layer of conductive paint applied to an insulating film surrounding the winding portions 10 and 12. Figure 3 will be discussed in greater,

detail in connection with a method for making the apparatus 2.

The presence of the shield 22 extends the high frequency range of the winding 16. Without this shield 22, the winding 16 would be capacitively coupled to the winding portions 1i) and 12 and the cores 4 and 6 thereby increasing the core losses and increasing the undesired inter-action between windings 16 and 8.

In the embodiment shown in Figure 4, a broken layer 34 of electrically conductive material forming a grounded electrostatic shield is placed between the winding portion 10a and the core 4a. A similar shield 35 is placed between the winding portion 12a and the core 6a, and a shield 36 is placed between the winding 16a and both of the cores 4a and 6a. The presence of any one of these shields 34, 35, or 36 increases the efiiciency and extends the high frequency range of all of the windings which are thereby electrostatically isolated from the core. For example, the shield 34 improves the operating characteristics of the winding portion 10a, and it also improves the characteristics of the control winding 16a which is thereby isolated from the core, which, due to poor Q of the dielectric constant of the material, introduces losses if the interwinding SSisleft' in' each of-the shielding layers 34 -and 35 in order to prevent them from acting as short-circuited turns around the cores. I'havefound that the use of these shields or conductive layers 34-and;35;increases the highfrequency limit of the windings 8aand16a and results in the improved operating characteristics -discus-sed'above. Conductive paintof the type sometimes used for printed electrical circuits is suitable for this purpose.

oil may befired'at a low temperature on the surface of the cores.

The shield "36; between the signal winding portions 1t a and 12a and the control winding 16a is formed of copper foil or a layer ofconductive paint carried by an insulating sheet can be 'used; An annular insulating gap 39'is provided topreventthis' shield from becoming a short-circuited turn. This shield36'improves the characteristics of the signal winding 8; by decreasing the re maining unbalance current, coupling capacity, and dielectric losses inthe windings and cores which are otherwise caused by changes in the'control current; especially at low permeabilities. Furthermore, the shield 36 isolates the winding 16a'from the winding 8 so that the control action therebetween takes place solely by virtue of changes in the magnetic conditions. of'the cores 4a and 6a.

Thelayers of conductive paint-34 and '35"may be replaced by thin layers of conductive foil wound around the cores daand 6a. respectively, with the edges thereof separated by small air gaps which serve to prevent short-circuit current in the shields. This arrangement is satisfactory for many purposes but generally is not so desirable as the conductive paint. Forione thing the foil can never be placed in such intimate contact with the cores as the conductive paint and accordingly an increased air gap necessarily exists between the windings and the cores.

InFigure 5, a layer of conductive foil 42is wound around two magnetic ceramic ring cores 4b and 6b and the signal windings 19b "and llb." This shielding layer has an S- shapedcross section and forms two' annular cavities or shields f43 and .44 which; respectively, house the cores 4b'and16b; Each of these cavities has a narrow annular space in the foil,' as at45 and46'so that the shield does notform ashOrt-circuitedfturn. The shield orjlayeroffoileg also isolates the controlwinding 16b frorn the signalywindin'g portions 19b and 12b, respectively, and atthe same time the central portion 48 of the layer 42 reduces the mutual *electrostatic'coupling between the winding portions 1% and 12b in the region between the cores 4b and6b; Ifidesired,fthe'cores 4b and 612 also can be coatedwith conductive paint as described in connection with Figure 4.

Figures 6 and 7 illustrate shielding apparatus to be used between two windings on the same core or between two windings either of which encompasses both cores. This shielding can beused in conjunction with any of the foregoing shielding arrangements when ,required. As before, two ring cores 4c and 60- (Figure 7 of ferromagnetic ceramic material are positioned in axial alignment. A winding 50-extends around the core 40 and may also encompass the core 60 or may be'connected in series with a similar'winding on core 60 immediately opposite the winding 50.

Thewinding 50 extends only part way around the core 40 so as to leave room for another winding 52;;whic h may be a radio frequency winding, as for-example an antenna winding. The winding '52 extends around the core 40 and may also encompass the core 60,-oritmay be connected in series with'another similar winding on the core 60 positioned directly opposite the Winding 52.-

Finelypowdered silver dispersed in asmall amount of essential In order to minimize the coupling between the Windings-50-and'52-an'S-shaped wire 54 is positioned on the cores 4c and 6c as shown in Figure 7. This S-shaped conductor is most conveniently made from a short length of ordinary copper wire, but other materials such as foil or the like can be used. This S-shaped shielding conductor is placed on the cores between one end of the winding 50 and the; adjacent endof the winding 52 as shown in Figure 6. Another S-shaped shielding condoctor 56 is positioned-on the cores betweenthe other ends of these windings.

A method for making saturable core apparatus, for example, such as the embodiment shown in Figures 1, 2, and 3 proceeds as follows. The cores 4 and 6 are each closely surrounded by layers or films of insulating material 23 and 24, respectively, which protect the cores 4 the cores 4 and 6 is extremely hard and tends to be some-=- whatrough: The insulation films 23' and: Z4; prevent rough places on the :cores .4 and 6 from cutting through;

the layers ofinsulatingenamel on the wires-inthe winding portions .ltlxandzlZ: There areseveral ways-in which these insulation films can be applied to the cores 4 and 6, for example,- as, by painting; spraying. dipping, =etc. using insulatingmaterial; in aliquid form-which is .thenallowed to harden. Howevenl have found that if the cores 4 and 6 are coveredwith fluid insulating. coating material, the penetration of thismaterial into the pores of the ferro: magnetic ceramic.coreslcausesxaloss .of effective Q. I have foundthat,apreferable method for forming this insulationeoating is toiwrap the cores tightly with a layer of thermosetting plastic typetape .which is suitably formed intoa continuous film of insulation by touring in an oven at elevated temperaturev Anexample of-such a tape. is

insulation.-layers:23 :and 24 :all'the. way around the ring cores 4sand 6,;as. shownin this embodiment. However, where the veryhighest-frequency :operation'is important, this insulatingtapemay belomitted from that portion of thering cores immediately adjacent: the radio-frequency winding -portions. ForL example, referring to Figure 7 for the purpose of makingthisexplanation clear, if the winding 52 is to. bexoperatedat; the very highest frequency, it is possible, to omit any insulating ;filmq-from the cores in the region,adjacent-winding 52, that .is,: for example between the.shields54vand 56. Insuchcase, a small air space'is provided by windin'gsformed with spacers on the cores whichgare removed;when ;the,winding is completed.

Afterthe. -insulation;layers 23;;and24 are cured in an oven the winding pertion s-;1tg) .-and-;l2 are Wound tightly around-the cores 4 ar d;6,;;and fthe two cores areplaced together vwit-hi an; insulating washes-:25 between the winding;portions;10-;and;12. In order, toinsulate the, outside of thewindings- 4 and 6 and to rig-idly'secure thetwo cores firmly together as a single-unit, a-third layer 26 of thermo setting insulating tape is then Wound over both'of thewinding portionsllland 12, and the unit 2'is again placed in an oven-at elevated temperature to curethe film 26.

into a suitable varnish orshellac and'allowed to dry. A

layer 27 of electrically'conductive point forming the shield 22 is then applied to cover substantiallythe :entire surface of the film 26, a small annular gap-28 being left in this paint layer 27 to prevent the circulation of short circuit" :3 current in the shield 22 around the cores 4 and 6. I have found that it is desirable to have the gap 28 arranged symmetrically with respect to all of the windings on the cores 4 and 6. Thus, this annular gap may be located around the inside of the shield 22 or around the outside thereof. I have found that it is usually preferable to locate this gap 28 on the inside of the modulator 2, leaving the peripheral portion of the shield 22 free to receive a grounding connection. A suitable conductive paint is that sol-d under the trade designation of DuPont silver dispersion No. 4817. Tightly around the outside of the shield 22 is wound one turn of a strip 29 of electrically conductive foil, which serves to provide a connection to the shield 22 whereby it may be grounded. The two ends of the strip 29 are pulled together and brought out from the modulator 2 to provide an external terminal for grounding. I have found that although any good conducting material may be used, silver foil is preferable because its oxide is conductive and hence a good electrical connection is always maintained betwen the strip 29 and the periphery of the shield 22.

In order to secure this strip to the shield and to insulate the shield from the winding 16, a fourth film of insulation is formed therebetween by winding a layer 30 of thermosetting insulating tape around the shield 22 and curing it in an oven. The winding 16 is then wound around the cores 4 and 6 and a final outside insulating film 31 is formed around the unit 2 by winding it once more with a layer 31 of thermosetting insulating plastic tape which is cured in an oven.

It is to be understood that the principles set forth above in connection with dual ring core structures applies equally Well to multiple ring core structures using more than two cores.

From the foregoing it will be apparent that the saturable core magnetic units embodying the invention are well adapted to attain the ends and objects set forth herein, that they are economical to manufacture, and that they can be modified readily so as to best fit the needs of each particular use.

- I claim:

1. A high-frequency saturable core magnetic apparatus comprising first and second ring cores of ferromagnetic ceramic material, a first winding including first and second winding portions around said first and second ring cores, respectively, and means connecting said winding portions in series, said second winding portion being wound in a reverse sense relative to said first winding portion so that the magnetic fields within said two ring cores caused by a current flowing through said first winding are in opposite directions, a second winding encompassing both of said cores, a substantially closed annular shield of electrically conductive material adjacent two of said winding portions and substantially completely encircling both said first and second cores, said shield being disposed between said first and second winding portions and said second winding and electrostatically isolating said winding portions from said second winding, said shield having a gap therein extending around the length of said annular shield to prevent the flow of short-circuit current therein, and a strip of electrically conductive material in contact with said annular shield and extending around the length of said annular shield, said shield including electrically conductive silver paint in contact with said strip.

2. A high-frequency saturable core magnetic apparatus according to claim 1 and including a second annular layer of electrically conductive material separating said second winding, said second annular layer having an insulating gap therein extending around the length of said ring cores, from said first Winding whereby said two windings are isolated and the high frequency range of said second winding is extended.

3. A high-frequency saturable core magnetic apparatus comprising first and second ring cores of ferromagnetic ceramic material, a first winding having first and second winding portions around said first and second ring c'or'es, respectively, and means connecting said winding portions in series, said second winding portion being wound in a reverse sense relative to said first winding portion so that the magnetic fields within said two ring cores caused by a current flowing through said first winding are in op posite directions, a second winding encompassing both of said cores, and a substantially continuous shield of electrically conductive material surrounding both of said cores and separating said first and second windings from each other, said shield having a substantially S-shape cross section and forming a pair of annular cavities, each of said cavities surrounding one of said cores, said shield having a pair of annular gaps, one of said gaps being in each of said cavity portions, for preventing the fiow of short-circuit current around either of said cavities.

A high-frequency saturable core magnetic apparatus according to claim 3 and wherein said shield has a central portion separating said first and second winding portions.

5. A high-frequency saturable core magnetic apparatus comprising first and second ring cores of ferromagnetic ceramic material, a first winding including first and second winding portions around said first and second ring cores, respectively, and means connecting said winding portions in series, said second winding portion being wound in a reverse sense relative to said first winding portion so that the magnetic fields within said two ring cores caused by a current flowing through said first winding are in opposite directions, a second winding encompassing both of said cores, and a first and a second 8- shaped shield of electrically conductive material, each of said shields forming two substantially closed loops, each one of said loops encircling one of said cores, said first and second shields being adjacent opposite ends of said first and second windings, whereby said windings are electrostatically isolated from each other by said S- shaped shields.

6. A high-frequency saturable core magnetic apparatus comprising first and second ring cores of ferromagnetic ceramic material, a first winding with first and second winding portions around said first and second cores, respectively, means connecting said winding portions in series, said second winding portion being wound in a reverse sense relative to said first winding portion so that the magnetic fields within said two ring cores caused by a current flowing through said first winding are in opposite directions, a first insulating film completely surrounding both of said winding portions and securing said cores together, an annular layer of electrically conductive material near the outer surface of said film, a gap in said conductive layer extending around the length of said annular layer, a strip of electrically conductive material in contact with said layer and extending around the length of said layer, said layer including electrically conductive silver paint in contact with said strip, a second insulating film surrounding said layer and said strip, and a second winding encompassing both of said cores and over lying said second film.

7. A high-frequency saturable core magnetic apparatus comprising first and second annular cores of ferromagnetic ceramic material, first and second films of insulating material covering substantially the entire surface area of each of said cores, each of said films comprising a winding of thermosetting tape placed upon one of said cores and then cured at elevated temperature including a first winding, first and second winding portions around said first and second cores, respectively, and overlying said insulating films, means connecting said winding portions in series, said second winding portion being wound in a reverse sense relative to said first winding portion so that the magnetic fields within said two annular cores caused by a current flowing through said first winding are in opposite directions, an insulating spacer between said winding portions, a third insulating film completely surrounding both of said winding portions and securing said cores together, said film comprising a winding of thermosetting I tape cured. at elevated temperature, an annular shield layer tof'ielectrically conductive material on theouter sur-' face of 'saidjth'i'r'd filrii, a gap in said 'annular layer and:

core portions, saidfi'rst, Winding including a first Winding 1 portion wound ,ar'oundsaid first core portion and a second winding portion wound around said second 'core portion, said second winding portionrbeingwound in a reverse sense rclativetosaid first windingportion so that the magnetic fieldswithingsaidtwoflcoreportions caused by a signal current flowingthrough said first Winding are in'opposite directions, a. second Winding magnetically coupled to each ofsaidcore. portions and ja'pair of substantially continuous-layers.of;electrica11y conductive silver paint closely adjacent allof the exposed, surfaces of each of said core Portions separating each of .said winding portions fromv its -vrespectivecore portiomeach of said layers of silver paint having a closed insulating line therein to prevent short-circuit current from flowing in said layers around said core portions, whereby the hysteresis and eddy current losses in said core portions are reduced and the high frequency range and frequency response of said apparatus is extended.

9- A high-frequency saturable core variable inductance apparatus 'comprisingfirst and second ferromagnetic ceramic-cores, a first winding around both-of said cores, said windingsincluding a first winding portion wound around said first core, a second winding portion Wound around said second core, said second Winding portion being wound in a reverse sense relative to said first winding portion sothatthe magnetic fields within said two cores caused bya signal current flowing through said first winding are-in opposite directions, a'second winding associated withportions of both :of said cores, and a pair of substantially continuous shields of electrically conductive material closely adjacent the exposed surfaces of each of said; cores near each of said first and second winding prtions,said.two shields as seen in cross section defining an S-shape, each :of-saidshields having an insulating 'gap therein to prevent sh rt-circuit current from flowing in said shield around saidcores, whereby the hysteresis and eddy current losses in saidcore pieces are'reduced and the high frequency range ,andfrequency response of said apparatus is extended.

10. A'high frequency saturable corevariable inductance apparatus comprising. a core portioncof ferromagnetic ceramic. material, a winding around said core portion, a

layer of electrically conductive paint applied directly' to" thepsu'rface of {said'icore portion; and a" second *windi'ng magnetically coupledto said core'portionr 11. A highffr'equencysaturable'corevariable inductance apparatus comprisinga core portion'of ferromagnetic 'ce-' ramic material defininga'substantially closed magnetically permeable path extending the length" of'said core; a'first winding on said fcore portion; a layer-of electrically conductive paint applied toth'e'surface of saidcore and substantially completely surroundingsaid coreportion, said layer havinga gap therein, extending along thelen'gth of' said core generally parallel to saidipath, and "a second" windingmagnetically coupled to'said core portion:

12; A high frequency saturable' corevariable inductance apparatus comprising first andsecond ferromagnetic'ceramic core portions, a. first winding aroundi one of said core portions, a second windingaround 'thdotherof'said core portions, said windings being connected in series with" said secondwinding being in a reverse sense relative to saidfirst winding so that the magnetic fields within saidtwo core portions caused by current'fiowingthrough' said first andsecond windings in series are inopposite directions, a third 'Winding magnetically coupled .to each of said core portions, and a layer of electrically conduc tive paint on the. exposed'surface of each of said'core portions.

13; A high frequencysaturable core variable inductance apparatus comprisingfirst and. second ferromagnetic ceramic core portions, a first windingaround one of said' core portions, a second winding around the other of said core portions, said windings being connected in series with said second winding being in a reverse sense relative to said first Winding so that the magnetic fields Within said two core portions caused. .by current flowing through said first andsecond windings in series are in opposite directions, a third winding magnetically coupled to each of said core portions arranged 'to control the magnetic saturation thereof, a layer of silver paint intermediate at least two of said windings, and a strip of silver foil in contact with said layer of silver paint.

References Cited in the file of this patent UNITED STATES PATENTS 1,567,797 Frank Dec. 29, 1925 1,586,970 Curtis June 1, 1926 1,673,331 Hess June 12, 1928 1,839,038 Kronmiller Dec. 29, 1931 2,164,383 Burton July 4, 1939 2,388,070 Middel Oct. 30, 1945 2,452,531 Snoek Oct. 26, 1948 2,488,370 Boelens et a1 Nov. 15, 1949 FOREIGN PATENTS 733,913 Germany Apr. 5, 1943 

